Add a generic, placement-aware mechanism for operators to inject HTML/JS
into every page that extends base.html or base_login.html. Each entry
takes name, enabled, placement (head_start | head_end | body_end), and
html. Replaces the need for per-vendor helpers when shipping feedback
widgets, analytics, or error-capture snippets.
Trust boundary mirrors the existing instance.logo_svg / instance.overview
pattern — admin-only, rendered with `| safe`. Resolved by
app/instance_config.py::get_custom_scripts(), surfaced in
/admin/server-config via _KNOWN_FIELDS["instance"]. Empty default keeps
the OSS vendor-neutral; sample Marker.io block ships commented out in
config/instance.yaml.example as the canonical example.
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